This invention is an assembly for a mold tool and specifically, a material compensation assembly for reducing sink mark formation on a molded article.
Typically, sink marks are the result of variations in plastic material cross-sections in a molded part. The thicker cross-section of molten material solidifies slower than surrounding thinner sections. Because plastic material decreases in volume during solidification, the thicker sections that solidify last pull in surrounding material to compensate for the decrease in volume. The pulling in of surrounding material will often result in sink marks forming on the outer surface. Slower solidification results in areas on the opposite surface that are inconsistent with the remainder of the outer surface. In other words, the outer surface includes depressions in the area of the increased material thickness.
The thicker section naturally shrinks more due to the additional material present in these localized areas. As appreciated, the last section to solidify is the very center of the thickest section. As this material shrinks, surrounding material is pulled in around and onto the thicker cross-section. Material that does not have the structural strength to resist this pull is deformed as it collapses in the direction of the pull, causing shrinkage. The end result is a surface sink mark or depression in areas of increased relative thickness.
Many molded parts have a “show” side that is desired to have a specific cosmetic appearance. Sink marks destroy the cosmetic appearance and degrade the aesthetic value of the plastic injection molded part. Currently, plastic injection molded parts are increasingly being used for exterior body panels in automotive applications. Further, injection molded parts are extensively used in consumer articles such as computers, monitors, telephones, radios, automotive parts and other consumer goods. In such applications, the appearance of the plastic injection molded part is of great importance. Sink marks complicate the fabrication of molds to produce such articles. These molds are complicated because they must be designed to eliminate or hide sink marks.
Accordingly, it is desirable to design an assembly to compensate that prevents depressions and sink marks caused by volume changes between adjacent areas of differing thickness.